1. Field of the Invention
The present invention relates to a technique for producing reduced metal agglomerates by heating and reducing metal oxide agglomerates containing a carbonaceous material using a moving-hearth heating furnace. Examples of the metal oxide agglomerates include agglomerates of a raw material containing iron oxides, nickel oxide, chromium oxide, cobalt oxide, or a mixture of these substances.
2. Description of the Related Art
As a method for making reduced iron, the Midrex process is well known. In this process, a reducing gas formed from natural gas is blown into a shaft furnace through a tuyere so that the shaft furnace is kept in a reducing atmosphere, and iron ore or iron oxide pellets charged in the furnace are reduced by being brought into contact with the reducing gas, and thereby reduced iron is obtained.
However, in this method, since natural gas, which is an expensive fuel, must be used to form the reducing gas and a large amount of natural gas must be supplied, an increase in production costs is inevitable.
Under these circumstances, recently, processes for producing reduced iron using relatively inexpensive coal instead of natural gas as the reducing material have been receiving attention again. For example, U.S. Pat. No. 3,443,931 discloses a process in which fine ore and a carbonaceous material (e.g., coal) are mixed together and pelletized, followed by reducing by heating in a high-temperature atmosphere, to produce reduced iron. In this process, dried iron oxide pellets containing a carbonaceous material are fed into a rotary hearth furnace at a given thickness, and the mixture is heated by radiant heat in the furnace while being moved in the furnace, and thereby the iron oxide pellets are reduced by the carbonaceous material. The reduced iron oxide pellets are radiation-cooled by a cooling plate, referred to as a chill plate, in the radiation cooling zone, and are then scraped away from the moving hearth by a discharge screw of a discharger and are discharged from the furnace.
In addition to the fact that the reducing material is coal-based, this process is advantageous over the Midrex process in that, for example, fine ore can be directly used, the reduction rate can be increased, and the carbon content in the product can be adjusted.
Although the process has the advantages described above, powder, which is generated from the iron oxide pellets due to various factors, such as rolling, friction, or dropping impact when the pellets are fed into the furnace, is also fed into the furnace together with the pellets. The fed powder is deposited on the moving hearth which rotates to form an iron oxide powder layer. Since the iron oxide powder layer includes the carbonaceous material, it is reduced in the same manner as the iron oxide pellets, and thus a reduced iron powder layer is formed. Although a portion of the reduced iron powder is discharged from the furnace by the discharger together with the reduced iron pellets, the other portion of the reduced iron powder remains on the moving hearth and is pressed against the surface of the moving hearth by the discharger. The reduced iron powder pressed against the surface of the moving hearth is deposited on the surface of the moving hearth without being reoxidized because of its denseness. Reduced iron powder is further added as the rotary hearth rotates and reduced iron powder is gradually integrated into the previously deposited reduced iron powder to form a reduced iron layer in the shape of a large plate. The plate-shaped reduced iron layer (hereinafter referred to as an xe2x80x9ciron platexe2x80x9d) may be scraped by the edge of the blade of the discharge screw and the separated reduced iron may be wound around the discharge screw or may prevent the reduced iron from being discharged because of clogging of the discharge port, giving rise to problems, such as shutdown.
A depression exists on the surface of the moving hearth after the iron plate is scraped off, and the charged agglomerates enter the depression. As a result, it is not possible to charge the agglomerates at a given thickness, the agglomerates cannot be heated homogeneously, and the rate of reduction varies for each agglomerate, resulting in a degradation in quality of the reduced iron.
Under these circumstances, in order to prevent the formation of the iron plate, the applicant of the present invention has carried out thorough research on the formation mechanism of the iron plate, and has completed an invention in Japanese Patent No. 3075721 (Prior Art 1). The above invention is characterized in that the operation is carried out by continuously or intermittently moving a discharger upward from the surface of a moving hearth, depending on the thickness of the iron oxide layer, so that a gap is provided between the surface of the moving hearth and the discharger. In the above invention, although the iron oxide powder layer formed on the moving hearth by powder mixed into the furnace together with the iron oxide pellets is reduced to form a reduced iron powder layer, the reduced iron powder layer is not densified because it, is not pressed by a discharger, such as a discharge screw, and the reduced iron powder layer is reoxidized during passing through the furnace again to form an iron oxide layer. Therefore, an iron plate is not formed.
As the discharger used in prior art 1 described above, a discharge screw having a schematic structure shown in FIG. 3 is generally employed.
That is, as shown in FIG. 3, a through-hole 26 is provided on the side wall of a moving-hearth furnace, and a screw axis 4 of the discharge screw is extended to the outside of the furnace and is supported by a screw axis bearing 24 arranged outside of the furnace. The screw axis 4 is revolved by a drive device for discharger 28 arranged outside of the furnace through a chain or the like. Since the discharge screw must be moved vertically during operation, an elevating device 22 for moving the screw bearing 24 vertically is provided, and an expansion joint 23, functioning as a gas-sealing means, which is made of metal is also provided so as to prevent air from entering the furnace through the gap between the through-hole 26 and the screw axis 4 and to prevent furnace gas from leaking out of the furnace.
However, in the metal expansion joint 23 as shown in FIG. 3, in general, since the amount of expansion in a direction perpendicular to the axial direction is smaller than the amount of expansion in the axial direction, it is difficult to secure the amount of vertical movement of the screw axis 4 required for the operation. Furthermore, as vertical movement is repeated, the expansion joint 23 is subjected to repeated elastic deformation in the direction perpendicular to the axial direction, and damage, such as cracks, due to metal fatigue easily occurs. When such damage occurs, in order to replace the expansion joint 23, the screw bearing 24 section must be disassembled by halting the operation, and thus the maintenance work is troublesome.
Although the case in which reduced iron agglomerates are produced using iron oxide agglomerates containing the carbonaceous material as raw materials by the rotary hearth furnace has been described above, even when raw materials including nonferrous metal oxides, such as nickel oxide, chromium oxide, and cobalt oxide, instead of iron oxides, are used as raw materials, it is possible to produce reduced metal by metallizing these oxides. However, in such a case, since a metal plate similar to the iron plate described above is also formed on the surface of the hearth, the formation of the metal plate must be prevented, thus giving rise to the same problems as those described above.
Accordingly, the objects of the present invention are to provide a moving-hearth furnace for producing reduced metal having a means for preventing a metal plate from being formed other than moving a discharger (discharge screw) vertically, so that the maintenance work can be significantly reduced, and to provide a method for operating the same.
In the present invention, a moving-hearth heating furnace includes a moving hearth which moves with a metal oxide-containing material being placed thereon, a heating furnace for heating the metal oxide-containing material to produce a heat-treated material while the moving hearth is moving in the heating furnace, and a discharger for discharging the heat-treated material from the heating furnace, wherein the moving hearth is movable vertically.
Further, in the present invention, the moving-hearth heating furnace includes an elevating device for moving the moving hearth vertically, the elevating device being provided on a supporting section for supporting the moving hearth.
The moving-hearth heating furnace can further comprise a seal plate provided around the entire lower section of the moving hearth and a water-sealing trough fixed on a side wall of the heating furnace, wherein the length of the seal plate and the depth and fixing position of the water-sealing trough are determined so that the lower end of the seal plate is kept being immersed in water in the water-sealing trough when the moving hearth is moved upward to the upper limit.
The moving-hearth heating furnace can further comprise a columnar partition provided on the moving hearth and a roof having a recess, wherein the top of the columnar partition is inserted into the recess and the height of the columnar partition and the depth of the recess are determined so that the top of the columnar partition does not come out of the recess when the moving hearth is moved downward to the lower limit.
In the present invention, a method for making reduced metal agglomerates includes the steps of feeding metal oxide agglomerates containing a carbonaceous material onto a moving hearth which moves in a heating furnace, heating and reducing the metal oxide agglomerates to produce reduced metal agglomerates while the moving hearth is moving in the heating furnace, and discharging the reduced metal agglomerates from the heating furnace by a discharger provided above and in close proximity to the moving hearth in the heating furnace. The moving hearth is continuously or intermittently moved vertically depending on the thickness of a metal oxide layer formed by the deposition of powder of the metal oxide agglomerates mixed into the heating furnace together with the metal oxide agglomerates so that a gap is provided between the surface of the metal oxide layer and the discharger during operation.
In the method for making reduced metal agglomerates, the rate of moving the moving hearth downward continuously or the amount of moving the moving hearth downward intermittently can be adjusted depending on the amount of powder of the iron oxide agglomerates entering the heating furnace
In the method for making reduced metal agglomerates, the rate of moving said moving hearth downward can be adjusted so that a gap corresponding to three-fourths or less of the average diameter of the agglomerates is provided between the edge of a blade of a discharge screw of the discharger and the surface of the moving hearth or the iron oxide layer.
In accordance with the present invention, since metallic powder generated by the reduction of powder of metal oxide agglomerates is not compressed into the surface of the moving hearth, the formation of a metal plate can be prevented. In addition, the maintenance workload for the sealing mechanism of the discharger can be significantly reduced, continuous operation is enabled for a longer period of time, and reduced metal having a high metallization rate can be obtained stably.